1. Field of the Invention
The present invention relates to a field effect transistor (hereinafter, referred to as FET) using a heterojunction epitaxial substrate and a manufacturing method thereof.
2. Description of the Related Art
Since an FET of which source/drain area of a heterojunction FET is formed in a self-aligning manner with respect to the gate electrode using ion implantation has only a small parasitic resistance in the source/drain, it is expected as a process which realize a high performance enhancement type FET (for example, refer to “J. K. Abrokwah et al, GaAs IC Symposium Digest, P127-130, 1993”).
In the above heterojunction FET, for the carrier supply layer which supplies electron to the active layer, or for the active layer in which no carrier supply layer is provided but the active layer itself is doped, a layer doped with Si is mainly used.
However, to activate an ion implanted area which serves as the source/drain area, a high temperature annealing process is indispensable. Due to the annealing process, there arises such problem as the heterojunction is changed in nature resulting in a deterioration of the FET characteristics. Conventionally, as a doping material for the N-type carrier supply layer or active layer itself of the epitaxial substrate used for the heterojunction FET, a Si (silicon) donor is mainly used. Since the Si donor is a dipolar impurity, due to the high temperature annealing process for activating the ion implanted area to form the source/drain area, the Si is displaced from the ordinary Ga grid position to the As grid position resulting in a decrease of activation. Further, due to the above-described high temperature annealing process, the Si donor forms compound matters with diffused F (fluorosis)-atoms, which adhered on the surface of the substrate in the process such as hydrofluoric acid processing, etching using CF4 gas, or the like, resulting in an inactivation. As a result, the carrier density decreases resulting in a deterioration of the FET characteristics. It has been reported by HAYAHUJI et al, that, particularly in hetero group of InAlAs/InGaAs, the Si donor is inactivated by F-atoms in the heat treatment at a temperature of around 400° C. (Appl. Phys. Lett., Vol. 66, P863-865, 1995). The inventor et al. has found that the above phenomenon occurs also in high temperature annealing process for activating ion implanted area.